The present invention relates to a radio communication system which includes a mobile station and a base station that communicate with each other over the air, and more particularly, to a radio communication system which improves the usability of a mobile station in an airport and the like.
A standard has been established for a multi-channel access (MCA) radio system for airports which plays an important role of ensuring the security of aircraft existing in airports and the comfort of users. For example, see a document entitled “AIRPORT DIGITAL MOBILE TELECOMMUNICATION SYSTEM,” ARIB STD T-87, October 2003 by Association of Radio Industries and Business (ARIB).
In a system conforming to the foregoing standard, an airport, which constitutes a basic unit, is composed of control station facilities, base station facilities, and mobile station facilities, wherein communications are made between a domestic terminal and a mobile station or between mobile stations (through a base station or directly) using a 400 MHz band. The system employs a quadruple TDM/TDMA scheme which separates a single radio carrier on a frame-by-frame basis and groups the frames into four slots for providing multiple channels. The system also employs a π/4 shifted QPSK modulation scheme. Specifically, a base station is assigned a single or a plurality of radio carriers (pair waves) from a 400 MHz band which is an available frequency band. Each of a downlink radio carrier f1 and an uplink radio carrier f2 is divided into frames which are sub-divided into slots which are then allocated to a control channel and a communication channel.
For special requirements of operational communications within an airport such as consistent security for aircraft, this system permits ensured emergency communications, preferential connections for crucial or important communications, and transmission function (hot mic function) for mobile stations. The foregoing standard also permits non-speech communications such as character communications, image transmissions, and the like through data communications, simultaneous speech and data communications, and the like in addition to speech communications. The hot mic function is also called a “hot mic remote monitor function” which allows the user to transmit a speech from a mobile station without pushing a PTT button (press-to-talk button). As the user pushes the PTT button down to enter a hot mic mode, the mobile station collects sound therearound which is transmitted to a destination mobile station connected thereto. Once in the hot mic mode, the destination mobile station is automatically limited to a reception function.
JP-A-2000-244631, for example, discloses an exemplary configuration of a conventional mobile station which makes digital mobile radio communications. This mobile station comprises a transmission/reception switching unit, a receiver, a demodulator, a codec, a speech processing unit, ear speakers, a microphone, a modulator, a transmitter, and a controller. Also, an operation panel and a display unit are connected to the controller. A loudspeaker is also connected to the controller, so that when the loudspeaker is drawn out of the mobile station, a speech of a communicating partner (the party on the other end) is switched from the ear speakers to the loudspeaker for emitting the speech at a higher volume, thus permitting the user to speak while viewing a screen.